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I teach at a school and I create PowerPoint presentations to use in my classroom. I also need to upload the PowerPoints to my school website for my students to access. Access is restricted and students must sign in with their school codes. Can I use images I find on Google search? Are all images copyrighted? If they do not have an "ownership" stamp on them, are they ok to use in this manner? Nothing is being sold and the presentations are taken down at the end of each year.